Sun tracking solar concentrator

ABSTRACT

A sun tracking solar concentrator includes a one or two-sided linear Fresnel lens imprinted on a rollable sheet that is curved to form as a cylindrical arc surface. A one sided lens has a first zero line or a center point that transmits sunlight through without any refraction. A two sided lens also has a second zero line that is perpendicular to the first zero line. The Fresnel lens may be spooled onto rollers at its two straight ends. The first zero line or the center point may be positioned along the cylindrical arc by rotating one or both of the rollers. This mechanism aimed at providing horizontal tracking of the sun as it moves from East to West. Vertical tracking is accomplished by a tiltable mount coupled to the two rollers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to solar concentrators, in particular to those that use flexible Fresnel lenses and track the movement of the sun.

2. Background

Most of the US landmass has solar potential varying between 3-8 kWh/m2 per day. If one could only convert 30% of the solar energy incident upon a typical 6 m×6 m two-car garage area in a state like Colorado, one could power the whole house, based on the average US daily consumption rate of 32 kWh and send an additional 16 kWh back into the grid for others every day. This potential remains to be realized. To date, much progress has been made, but the amount of electricity generated from solar technologies remains very low.

The two main technologies for harvesting solar power are (1) photovoltaics and (2) solar thermal. Photovoltaic devices convert solar energy directly to electricity. Solar thermal devices concentrate and convert the solar energy to heat which is then converted to electricity.

Photovoltaic devices can be in the form of flat panels that are exposed to sunlight or concentrated photovoltaics (CPV) systems that employ sunlight concentrated onto photovoltaic surfaces. Concentrating solar energy leads to increased efficiency in photovoltaics (from 15% to 38.5%) and reduces costs since much less photovoltaic device area is required. Concentration ratios can range from 2 to 800 times, i.e. 2-800 suns. Likewise, in solar thermal systems, the sun's rays must be highly concentrated (100-1000 suns) for efficient electric power generation. Both systems use optical techniques to focus incident sunlight into a small beam. Higher concentration generally means more efficient power generation. Moreover, in a high concentration design, tracking is critical to keep the sunlight focused onto the small solar cell or a hot spot.

The three main categories of existing technologies for solar concentrators are parabolic troughs, dish reflectors, and Fresnel lenses and reflectors. Parabolic troughs concentrate incoming light along one dimension leading to a line of concentrated light. Parabolic dishes, on the other hand, concentrate along two dimensions. Fresnel lenses and reflectors can be linear resulting in one dimensional concentration or radial leading to two dimensional concentration. Concentration along two dimensions is required for use with CPV solar cells to make them cost effective. This makes radial Fresnel lenses and dish reflectors the main candidates. Dishes are made of metals which make them expensive and heavy and thus unsuitable for distributed applications. Radial lenses are inexpensive and are the concentrator of choice for home-owner solar technology. There are radial Fresnel lenses on weather-tough acrylic currently available on the market. Even though these lenses are light and inexpensive, external moving parts are needed to orient them to track the sun which raises the cost of the end product significantly. The trackers are also large and bulky and thus not suitable for many applications, especially distributed applications, such as those set up in remote locations or camps, mounted on rooftops, or installed in backyards.

Both photovoltaic systems and solar thermal systems would benefit greatly from sun tracking solar concentrators built with common inexpensive and lightweight materials. The present invention is aimed at addressing this need.

SUMMARY OF THE INVENTION

The present invention is directed toward a sun tracking solar concentrator which includes a single or double layer linear Fresnel lens that can be used with a solar thermal or photovoltaic energy conversion system. In case of a single layer linear Fresnel lens, the rays of the sun are concentrated along one dimension onto a narrow line straddling the focal line of the lens. In case of the double layer Fresnel lens, the solar rays are concentrated along two dimensions onto a spot centered at the focal point of the lens.

The surface of the Fresnel lens is curved to form a cylindrical arc surface. During use, the axis of the cylinder is preferably positioned such that it lies substantially in the plane perpendicular to the East-West axis. The East-West axis may be defined as the line that connects the two points on the horizon, the first point being where the sun rises and the second being the point where the sun sets on the Spring and Autumn equinoxes.

The linear Fresnel lens surface has a first zero line substantially parallel to the cylinder axis where the solar rays incident upon it pass through with little or no refraction perpendicular to the cylinder axis. The solar rays incident at other locations are refracted perpendicular to the cylinder axis by the chain of prisms of the Fresnel lens towards the focal area.

The linear Fresnel lens surface may also have a second zero line substantially perpendicular to the cylinder axis where the solar rays incident upon it pass through with little or no refraction parallel to the cylinder axis. The solar rays incident at other locations are refracted parallel to the cylinder axis by the chain of prisms of the Fresnel lens towards the focal area.

Based on the described geometry, tracking the sun may be accomplished by accommodating two angles: First is the azimuth angle as the sun travels from East to West during the day. The second is the elevation angle as the sun rises, traverses its daily path overhead, and sets. The azimuth and elevation tracking mechanisms maintain the solar concentrator in its preferred orientation, which is achieved when the Fresnel lens is positioned such that a plane tangent to its surface and passing though the first zero line is substantially perpendicular to the incident solar rays.

The sun tracking solar concentrator may exhibit local invariance of the angle of incidence as long as the preferred orientation of the solar tracker is maintained. This local invariance renders the angle of incidence of solar rays at any given point on the solar concentrator lens substantially constant despite the movement of the sun across the sky as long as the preferred orientation is maintained. The substantially constant angle of incidence allows for the optimization of the cylindrical Fresnel lens design prism by prism resulting in enhanced optical efficiency.

Accordingly, an improved sun tracking Fresnel lens solar concentrator is disclosed. Advantages of the improvements will appear from the drawings and the description of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals refer to similar components:

FIG. 1A schematically illustrates the Fresnel lens concept as known in the prior art;

FIG. 1B schematically illustrates a linear Fresnel lens as known in the prior art;

FIG. 1C schematically illustrates a radial Fresnel lens as known in the prior art;

FIG. 1D schematically illustrates a section of the Fresnel lens prism chain with nomenclature conventions as known in the prior art;

FIG. 1F schematically illustrates a linear Fresnel lens with support structure to hold or suspend the lens as a cylindrical arc surface as known in the prior art;

FIG. 1G schematically illustrates the underside of the linear Fresnel lens shown in FIG. 1F;

FIG. 1H schematically illustrates the rays refracted as they pass through the linear Fresnel lens shown in FIG. 1F;

FIG. 2 schematically illustrates the movement of the sun across the sky on different days of the year as known in the prior art;

FIG. 3 schematically illustrates a cylindrically shaped linear Fresnel lens that concentrates sunlight along one dimension and utilizes an external pan and tilt mechanism to track the sun as known in the prior art;

FIGS. 4A and 4B schematically illustrate a first sun tracking solar concentrator that concentrates sunlight along one dimension;

FIGS. 5A and 5B schematically illustrate a second sun tracking solar concentrator that concentrates sunlight along two dimensions;

FIG. 6 schematically illustrates a first modified sun tracking solar concentrator;

FIG. 7 schematically illustrates a second modified sun tracking solar concentrator;

FIG. 8A schematically illustrates a third embodiment of a sun tracking solar concentrator;

FIG. 8B schematically illustrates the folded view of the solar concentrator shown in FIG. 8A;

FIGS. 9A and 9B schematically illustrate a rolling mechanism for a sun tracking solar concentrator;

FIGS. 10A, 10B, and 10C schematically illustrate the local invariance of the angle of incidence for a sun tracking solar concentrator; and

FIGS. 11A and 11B schematically illustrates a bundle of solar rays that pass through a single prism of a Fresnel lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning in detail to the drawings, FIG. 1A illustrates in a series of steps (a)-(d) how a Fresnel lens 10 can be constructed by collapsing a continuous surface plano convex lens 20 into an equivalent power Fresnel lens 10. This concept is well known to those skilled in the art. As shown, the Fresnel lens consists of a chain of prisms. Commonly, the prisms can be arranged linearly in rows or radially in concentric circles. A linear arrangement shown in FIG. 1B is commonly called a linear Fresnel lens 12 and provides one dimensional concentration of incident sunlight 90 onto an often narrow rectangular area 14 around the focal line of the lens. A radial arrangement shown in FIG. 1C is commonly called a radial Fresnel lens 16 and provides two dimensional concentration of incident sunlight 90 onto a small rectangular area 18 around the focal point of the lens.

FIG. 1D shows a section of the Fresnel lens prism chain with nomenclature conventions. The prism has two facets, called the slope facet 30 and the draft facet 32. The distance between the peaks of the prisms is facet spacing 34. The angle between the line parallel 38 to the plano surface 40 of the Fresnel lens and the slope facet 30 is the slope angle 36. The angle between the line 42 perpendicular to the plano surface 40 of the Fresnel lens and the draft facet 32 is the draft angle 44.

FIG. 1F illustrates a linear Fresnel lens 100 with support structure 106 to maintain a cylindrical arc shape. The support structure 106 may consist of ribs, wires or struts that maintain the cylindrical shape. The cylindrical outer surface 110 of the lens 100 pointed towards the sun is smooth. The chain of prisms of the Fresnel lens 100 are situated on the inside surface 120. Alternately, the chain of prisms of the Fresnel lens 100 may be situated on the outside surface 110 with the inside surface 120 being smooth. The material of the lens can be a flexible transparent polymer. If the material is thin and has a tendency to bow or sag, it can be held under slight tension. FIG. 1G illustrates a small cutout of the Fresnel lens with the outer surface 110′ from another perspective, showing the linear arrangement of prisms 130 and the internal surface cutout area 120′. The lens concentrates incident sunlight 90 in one direction around a focal line 140. As shown in FIG. 1H, the rays of the sun incident on the first zero line 150 go through the cylindrical Fresnel lens with little or no refraction. The remaining rays incident on the outer surface 110 are refracted towards the focal line 140. Typically, the ratio of the area of the linear Fresnel lens 100 surface to the area of the focal line 140 area is between 10 and 100. There are numerous examples of this type of solar concentrator in the art. Many of these solar concentrators are made of polymethyl methacrylate (PMMA) which is a transparent thermoplastic sometimes called acrylic glass.

FIG. 2 illustrates the movement of the sun across the celestial sphere 210 for four days during the course of a year for a location in the Northern hemisphere near latitude 35. The three solar paths shown are the solar path on Summer solstice 220, the solar path on Winter solstice 222, and the solar path on the Spring and Autumn equinoxes 224. Two angles define the position of the sun. These angles are the elevation angle 230 and the azimuth angle 240. Tracking the sun is important for solar concentrators. The solar concentrators often have a preferred orientation with respect to the sun. This preferred orientation is the one which substantially maximizes the amount of solar energy that can be concentrated and later converted to other forms of energy. Thus, many solar concentrators are mounted onto solar trackers that cause the solar concentrators to assume their preferred orientations with respect to the incident solar rays 90.

FIG. 3 illustrates the cylindrical linear Fresnel lens 100 with support structure 106 (shown earlier in FIG. 1F) mounted onto a solar tracking pan tilt mechanism 310. The pan tilt mechanism 310 is used to maintain the preferred orientation of the Fresnel lens 100 with respect to the incident solar rays 90. The preferred orientation is when the cylindrical Fresnel lens 100 is positioned such that a plane 320 tangent to the surface 110 of the cylindrical Fresnel lens and passing though the first zero line 150 of the cylindrical Fresnel lens 100 is substantially perpendicular to the rays 90 of the sun. This serves two purposes: First, the solar rays 90 are focused substantially at the same focal line 140 regardless of what the azimuth angle 240 of the sun or the elevation angle 230 of the sun is. Second, the solar energy concentrated along the focal line 140 is substantially maximized. The pan adjustment 340 is substantially coupled to the sun's azimuth angle 240; whereas, the tilt adjustment 372 is substantially coupled to the sun's elevation angle 230. There are numerous examples of this type of solar tracking in the art.

FIG. 4A illustrates a sun tracking solar concentrator 400 which includes a thin cylindrical linear Fresnel lens 410 spooled onto rollers 430. The rollers 430 rotate to position the first zero line 450 of the lens along the cylindrical surface of the lens. The support structure 416 which maintains the cylindrical surface of the lens. The support structure 416 may also couple the rollers 430 so that the rollers 430 can pivot around a common axis. This pivoting allows for a tilt adjustment of the midline 450. Alternately, the support structure 416 may be mounted onto a tilt mechanism 470. FIG. 4B shows the rollers 430 and the Fresnel lens 410 in greater detail.

As with the Fresnel lens 100 of FIG. 3, the preferred orientation is when the cylindrical Fresnel lens 410 is positioned such that a plane 420 tangent to the surface of the cylindrical Fresnel lens 410 and passing though the first zero line 450 of the cylindrical Fresnel lens 410 is substantially perpendicular to the incident solar rays 90.

The rollers 430 rotate to position the first zero line 450 so that the cylindrical Fresnel lens assumes the preferred orientation. The tilt adjustment 472 may also be necessary to assume the preferred orientation. The tilt adjustment 472 is substantially coupled to the sun's elevation angle 230 whereas the positioning of the first zero line 450 by way of turning the rollers 430 is substantially coupled to the sun's azimuth angle 240.

For the thin Fresnel lens to conform to a spool 432, its material may need to be sufficiently rollable and thin. One suitable material is a plastic sheet made from the resin polyethylene terephthalate (PET). Another generic term for this material is polyester film or plastic sheet. Also, some people refer to it as Mylar®, which is a registered trademark of Dupont Tejjin Films. A Fresnel lens may be imprinted onto a plastic sheet using one of many well-known methods in the art, such as hot-press embossing.

FIG. 5A illustrates another sun tracking solar concentrator 500 which includes a thin two sided cylindrical linear Fresnel lens 510 spooled onto rollers 530. FIG. 5B shows the cylindrical linear Fresnel lens 510 flattened and in greater detail. The rollers 530 position the first zero line 550 of the lens along the cylindrical surface of the lens. The chain of prisms on the inside of the cylindrical surface bends the incident rays towards the first zero line 550 whereas the chain of prisms on the outside of the cylindrical surface focuses incident rays towards the second zero line 552. The result is two dimensional concentration of incident light onto concentration spot 540. Typically, the ratio of the area of the surface of linear Fresnel lens 510 to the area of the concentration spot 540 is between 100 and 1000.

The support structure 516 of this sun tracking solar concentrator 500 maintains the cylindrical surface of the lens is mounted onto a tilt mechanism 570. The tilt adjustment 572 is substantially coupled to the sun's elevation angle 230. The positioning of the first zero line 550 by way of turning the rollers 530 is substantially coupled to the sun's azimuth angle 240. As with the Fresnel lens 100 of FIG. 3, the preferred orientation is when the cylindrical Fresnel lens 510 is positioned such that a plane tangent to the surface of the cylindrical Fresnel lens 510 and passing though the first zero line 550 of the cylindrical Fresnel lens 510 is substantially perpendicular to the incident solar rays 90.

Sun tracking solar concentrators may also incorporate Fresnel lens materials which do not permit spooling around rollers. FIG. 6 and FIG. 7 illustrate two such modifications. In the sun tracking solar concentrator 600 shown in FIG. 6, the Fresnel lens is not spooled onto the rollers 630. Instead, the two rollers 630 are part of a mechanism used to position the first zero line 650 of the Fresnel lens 610 along the cylindrical arc between lines 660 and 670. The bottom surface 680 of the support structure 116 can be mounted onto a tilting platform to provide tilt adjustment. In the sun tracking solar concentrator 700 shown in FIG. 7, the rollers are eliminated altogether. The Fresnel lens 710 is shaped into a cylinder. The cylinder is rotated around its axis 720, to position the first zero line 750 of the Fresnel lens along the cylindrical arc between lines 760 and 770. The bottom surface 780 of the support structure 716 can be mounted onto a tilting platform to provide tilt adjustment. The two sun tracking solar concentrators 600, 700 may provide one or two dimensional concentration. One dimensional concentration can be achieved with one sided Fresnel lens and two dimensional concentration can be achieved with a two sided Fresnel lens as shown in the sun tracking solar concentrator 400 and the sun tracking solar concentrator 500, respectively. The location of concentrated sunlight is not shown to keep the illustrations uncluttered.

FIG. 8A illustrates another sun tracking solar concentrator 800. Four cylindrically mounted strips 810, 812, 814, 816 of a thin two sided linear Fresnel lens are spooled onto rollers 830 and 832 which rotate to position the common first zero line 850 of the lens strips 810, 812, 814, and 816 along the cylindrical surface of the composite lens. The chain of prisms on the inward facing side of the cylindrical surface bends the incident rays towards the common first zero line 850 whereas the chain of prisms on the outward facing side of the cylindrical surface focuses incident rays towards the common second zero line 852. The result is two dimensional concentration of incident solar rays 90 onto concentration spot 840. The support structure 860 that maintains the cylindrical surface of the lens is mounted onto two legs 866 and 868 that are further coupled to two hydraulic cylinders 876 and 878 which collectively serve as the tilt mechanism. A heat exchange engine 842 is thermally coupled to the concentration spot 840. The heat engine may be a Stirling engine producing electrical output.

The support structure 860 is shown with five ribs that maintain the cylindrical shape of the Fresnel lens strips 810, 812, 814, and 816. The support structure 860 may contain further supporting beams or braces to reinforce its strength against external forces, e.g., wind. The support structure 860 may also be equipped by mechanisms that allow it to be stowed when wind speeds exceed safe levels. The support structure 860 may also be built so it can be folded for easy transport or storage as shown in FIG. 8B. Having a folding support structure 860′ is also convenient for reducing assembly complexity, labor, and time. The dimensions of the Fresnel lens strips 810, 812, 814, 816 of the sun tracking solar concentrator 800 determine the solar collection area and hence the energy output from the Stirling engine. It is expected that an area of approximately 10 square meters can be used to generate 1.6 kW of peak electrical power. This size can be achieved with approximately 50 cm wide strips that roll across ribs with arclengths of approximately 5.5 meters.

FIG. 9A illustrates one mechanism for rolling the Fresnel lens strip 812 in FIG. 8A to position the common first zero line 850 at the desired location. As shown in FIG. 9B, the Fresnel film strip 812 contains two rows of perforations 920 and 922 along its length positioned at the top and bottom of the strip 812. The two rows of perforations 920 and 922 are used for transporting and steadying the strip 910. They are locked onto two rows of sprockets 930 and 932 positioned on two chains 940 and 942. As the two chains 940 and 942 roll across the two gears 950 and 952, the film 812 is spooled from one roller 830 to the other roller 832 or vice versa. The rows of sprockets 930 and 932 hold the film strip 812 in slight tension to be suspended over the concentration spot 840. The rows of sprockets 930 and 932 also prevent the film strip 812 from rubbing against the ribs. A cover may be added to secure the rows of sprockets 930 and 932 in the rows of perforations 920 and 922 and thus prevent the strip 812 from coming loose.

FIGS. 10A, 10B, and 10C collectively illustrate a common property of the sun tracking solar concentrators 400, 500, 600, 700, 800 described above. This common property is referred to herein as “local invariance of the angle of incidence.” This property can be summarized as follows: The angle of incidence 1060 of solar rays 90 at any single point 1011 on the Fresnel lens 1010 remains substantially constant despite the movement of the sun across the sky provided that the preferred orientation of the Fresnel lens 1010 is maintained. As mentioned earlier, the preferred orientation is when the Fresnel lens 1010 is positioned such that a plane tangent to its surface and passing though the first zero line 1050 is substantially perpendicular to the rays 90 of the sun. The substantially constant angle of incidence 1060 at any single point 1011 allows for the optimization of the Fresnel lens 1010 design prism by prism as explained further below.

FIG. 11A illustrates a bundle of solar rays 1190 that are incident on a region 1114 of the cylindrical Fresnel lens 1110. As shown in further detail in FIG. 11B, the region 1114 is located between surface normal lines 1116 and 1118 of the Fresnel lens 1110. The bundle of solar rays 1190 incident upon the region 1114 passes through substantially only a single prism 1112. The facet spacing 1134 of the prism 1112 is small enough that the angles of incidence 1160 of all the rays in the solar ray bundle 1190 are substantially equal to another. The solar ray bundle 1190 is refracted first as it enters the surface of the Fresnel lens 1110 and second as it exits the prism 1112. The desired angles of refraction 1170 for all rays in the solar ray bundle 1190 are also substantially equal to another. Knowing the angle of incidence 1160 as well as the desired angle of refraction 1170, both measured with respect to the surface normal 1116 (or 1118) of the Fresnel lens 1110 makes it possible to optimize the design parameters of the prism 1112. These design parameters are facet spacing 1134, slope angle 1136, and draft angle 1144.

One design process that takes advantage of the property of constant angle of incidence may be described as follows:

-   -   STEP 1. Select material of the Fresnel lens. This will determine         the refractive index.     -   STEP 2. Select the thickness of the Fresnel lens.     -   STEP 3. Select the Fresnel lens focal length, f number,         cylindrical geometry, and dimensions.     -   STEP 4. Determine the maximum operational curvature of the         cylindrical Fresnel lens. This is the angle of the arc which is         endowed with Fresnel prisms. Its value is generally between 90         degrees and 180 degrees.     -   STEP 5. Formulate the initial design for the Fresnel lens. This         design will be optimized.     -   STEP 6. Divide the aperture of the Fresnel lens into segments         each of which correspond to a single prism path for the incident         solar rays.     -   STEP 7. Determine the prism inclination and the angle of         incidence of solar rays per each segment.     -   STEP 8. Determine the design parameters of the prism per each         segment.

These steps can be iterated as needed. As already mentioned in the text description associated with FIGS. 11A and 11B, the angles of incidence 1160 and the angles of desired refraction 1170 for all rays in the solar ray bundle 1190 are substantially equal. Any small differences between these angles can be taken into account for further optimizing the design parameters of the prism 1112.

Finally, a two layer Fresnel lens for concentrating solar rays 90 along two dimensions may be replaced with a one layer Fresnel lens, by arranging the chain of prisms of the Fresnel lens radially. It has already been mentioned in the text referencing FIG. 1C that a radial arrangement—commonly called a radial Fresnel lens 16—provides two dimensional concentration of incident sunlight 90.

FIG. 12A illustrates the Fresnel lens 1200 with the radial arrangement of the chains of prisms around a center 1250. The lens 1200 is laid flat. FIG. 12B illustrates the cylindrical arc shape in which the Fresnel lens 1200 is to be deployed when used as part of a solar concentrator of this invention. Referring again to the cylindrical arc shape of the Fresnel lens 1200 of FIG. 12B, solar rays incident upon the center 1250 pass through with little or no refraction. The solar rays incident at other locations are refracted by the chain of prisms of the Fresnel lens 1200 towards the concentration spot 1240.

The preferred orientation of the Fresnel lens 1200 is achieved when the Fresnel lens 1200 is positioned such that a plane tangent to its cylindrical arc surface and passing though the center 1250 is substantially perpendicular to solar rays 90. The center of the Fresnel lens 1200 can be positioned along the cylindrical arc as previously described above for the sun tracking solar concentrators 400, 500, 600, 700, 800.

Thus, a sun tracking solar concentrator is disclosed. While embodiments of these inventions have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The inventions, therefore, are not to be restricted except in the spirit of the following claims. 

1. A sun tracking solar concentrator comprising: a Fresnel lens comprising a linear arrangement of prisms with a common non-refracting first zero line, the Fresnel lens being curved to form a substantially cylindrical arc surface such that the non-refracting first zero line is parallel to a cylinder axis; and a mechanism for positioning the non-refracting first zero line along the cylindrical arc surface and parallel to the axis of the cylinder.
 2. The sun tracking solar concentrator as in claim 1 wherein the mechanism positions the non-refracting first zero line so that a plane tangent to the cylindrical arc and including the first zero line is substantially perpendicular to incident solar rays.
 3. A sun tracking solar concentrator comprising: a Fresnel lens comprising a radial arrangement of prisms with a common non-refracting center point, the Fresnel lens being curved to form a substantially cylindrical arc surface; and a mechanism for positioning the non-refracting center point on the cylindrical arc surface.
 4. The sun tracking solar concentrator as in claim 3 wherein the mechanism positions the non-refracting center point so that a plane tangent to the cylindrical arc and including the non-refracting center point line is substantially perpendicular to incident solar rays.
 5. A sun tracking solar concentrator comprising: a single sided Fresnel lens comprising a linear arrangement of a chain of prisms with a common non-refracting first zero line, the single sided Fresnel lens being curved to form a substantially cylindrical arc surface such that the non-refracting first zero line is parallel to the cylinder axis; and a mechanism for orienting the non-refracting first zero line so that a plane tangent to the cylindrical arc and including the first zero line is substantially perpendicular to incident solar rays.
 6. A sun tracking solar concentrator comprising: a two sided linear Fresnel lens, comprising a chain of prisms with a common non-refracting first zero line on a first side for concentrating rays of the sun along a first axis and a chain of prisms on a second side for concentrating rays of the sun along a second axis perpendicular to the first axis, the two sided linear Fresnel lens being curved to form a substantially cylindrical arc surface such that the non-refracting first zero line of the first side is parallel to the axis of the cylinder; and a mechanism for orienting the non-refracting first zero line of the first side so that a plane tangent to the cylindrical arc and including the first zero line is substantially perpendicular to incident solar rays.
 7. The sun tracking solar concentrator as in claim 6 wherein the first side of Fresnel lens faces inward and the second side faces outward.
 8. The sun tracking solar concentrator as in claim 6 wherein the Fresnel lens is imprinted on a rollable sheet.
 9. The sun tracking solar concentrator as in claim 8 wherein the Fresnel lens is spooled onto rollers.
 10. The sun tracking solar concentrator as in claim 9 wherein the rollers are rotated to position the non-refracting first zero line of the first side along the cylindrical arc.
 11. The sun tracking solar concentrator as in claim 9 wherein the rollers are tilted to orient the non-refracting first zero line of the first side with respect to the ground.
 12. The sun tracking solar concentrator as in claim 9 wherein the rollers are coupled to each other and pivot around a common axis to orient the non-refracting first zero line of the first side with respect to the ground.
 13. A sun tracking solar concentrator comprising: a linear Fresnel lens imprinted onto a rollable sheet and comprising a chain of prisms with a common non-refracting first zero line on one side of the sheet for concentrating rays of the sun along a single dimension, the linear Fresnel lens being curved to form a substantially cylindrical arc surface such that the non-refracting first zero line of the linear Fresnel lens is parallel to the cylinder axis, and a pair of rollers onto which the Fresnel lens is spooled, the rollers being configured to rotate to position the non-refracting first zero line.
 14. The sun tracking solar concentrator as in claim 13 further comprising a tiltable mount coupled to the pair of rollers.
 15. A sun tracking solar concentrator comprising: a two sided linear Fresnel lens imprinted onto a rollable sheet and comprising a first chain of prisms with a common non-refracting first zero line on a first side of the sheet for concentrating the rays of the sun along a first axis and a second chain of prisms on a second side of the sheet for concentrating rays of the sun along a second axis perpendicular to the first axis, the two sided linear Fresnel lens being curved to form a substantially cylindrical arc surface such that the non-refracting first zero line of the first chain of prisms is parallel to the cylinder axis; and a pair of rollers onto which the two sided Fresnel lens is spooled, the rollers being configured to rotate to position the non-refracting first zero line of the first side.
 16. The sun tracking solar concentrator as in claim 15 further comprising a tiltable mount coupled to the pair of rollers.
 17. The sun tracking solar concentrator as in claim 15 further comprising at least a pair of ribs configured to curve the two sided Fresnel lens substantially into the cylindrical arc surface.
 18. The sun tracking solar concentrator as in claim 17 further comprising sprockets traversing along the ribs and coupled into perforations formed along a curved side of the lens, the sprockets maintaining the Fresnel lens in the shape of the cylindrical arc surface under tension.
 19. The sun tracking solar concentrator as in claim 15 wherein the two sided linear Fresnel lens is cut into at least two strips that run perpendicular to the non-refracting first zero line of the first side and each strip is spooled onto a pair of rollers.
 20. The sun tracking solar concentrator as in claim 19 further comprising at least three ribs configured to curve the strips substantially into the cylindrical arc surface.
 21. The sun tracking solar concentrator as in claim 20 further comprising sprockets traversing along the ribs and coupled into perforations formed along curved sides of the strips, the sprockets maintaining the strips in the shape of the cylindrical arc surface under tension. 